The invention relates to a chainsaw.
DE-A1 21 327 47 discloses a generic chainsaw in which, between a rotational member and a cam, a pre-stressed torsion spring is disposed that seeks to rotate the cam resting against the stop in such a rotational direction that the blade is constantly subjected to a displacement force in the longitudinal direction. In this manner, the blade is kept at the greatest possible distance from the sprocket. This causes a tension force to constantly act on the saw chain, compensating for an increase in the length of the saw chain resulting from wear and temperature influences during operation.
During assembly, the torsion spring is tensed by means of the rotational member, which is embodied as a turning knob, until the cam rests against the stop with sufficient pre-tension, thus achieving the optimal chainsaw tension.
In a chainsaw with a chain tensioning device known from WO 98/33631, the blade is fixed between two securing plates and, together with these plates, is secured in a longitudinally movable fashion to the housing. A bolt, which extends through a bore in one cam and an oblong hole in the housing, is screwed into the one holding plate, which is guided on the housing in a longitudinally moveable manner, and fixes the cam to the housing by means of its bolt head. For the purpose of tensing the saw chain, the bolt is loosened and the cam is rotated so that the bolt, together with the holding plate, is moved in the direction of the blade tip. The rotation of the cam should be stopped once the saw chain has attained the desired tension. The bolt must then be tightened again so that the cam and the holding plates, together with the blade, are fixed to the housing in a frictional, non-positive fashion in the set position.
The known embodiments for tensing the saw chain are relatively expensive and involve the danger that the positional locking of the blade may slacken, with the chain tension decreasing as a result. This increases the danger that the saw chain may fly off the blade. This can only be prevented by frequently retensioning the chain.
The chainsaw according to the invention, has the advantage that a position of the blade in relation to the sprocket selected by the chainsaw user can be secured in a form-fitting fashion and only has to be readjusted if the chain has lengthened as a result of wear or if the blade and the sprocket and/or the deflection wheel have been worn down. Here, the chain tensioning device can be operated manually without any tools. For this purpose, it is merely necessary tl alternately rotate two handwheels that are disposed concentrically and Immediately adjacent to one another, each in the same direction. An automatic slackening of the chain pre-stressing device with the result of decreasing chain tension during operation of the chainsaw is reliably prevented by ability to the positionally lock the cam disc in a form-fitting fashion by means of the handwheels because the blade is thus held fixed in the set tension position.
By virtue of the fact that the rotational member is comprised of two shell-like handwheels that can be slid one into the other and rotated in relation to one another around a common axis, a convenient operation is produced with which the saw chain can be tensed without tools using one hand, in a xe2x80x9cblindxe2x80x9d fashion, i.e. without the user having to search for the control elements.
By virtue of the fact that the inner handwheel can be axially displaced in relation to the housing and can be supported on the blade, the blade can be locked in a definite position, i.e. without the possibility of user error, particularly after the chain tension has been set to the desired value beforehand using the outer handwheel.
By virtue of the fact that the outer handwheel is coupled to the cam in a rotation-transmitting manner, in particular by way of an internal spline/external spline connection, a secure transmission of rotation to the cam is assured, with the connection between the outer handwheel and the cam being particularly simple to produce and assemble, due to the fact that it can have coarse tolerances.
By virtue of the fact that the cam is mounted so that it can be moved axially and elastically in relation to the housing and/or the handwheel, after the cam has been rotated a maximum of three-quarters of a rotation, it locks into its working position in relation to a stop plate drive-connected to the blade. In this manner, the assembly of the chain tensioning device is simple and safe from user error, e.g., after the saw chain has been changed.
By virtue of the fact that the outer handwheel can, by way of a detent coupling, be coupled to the housing, in particular to the sprocket cover, an undesired change in the saw chain tension during the adjustment process is prevented.
By virtue of the fact that the detent coupling is preferably embodied as a radial detent coupling, the rotational position of the outer handwheel can be locked with the greatest possible locking action. In this instance, this locking position can be established in a form-fitting fashion by means of the inner handwheel in such a way that it cannot be changed unintentionally during operation of the chainsaw.
By virtue of the fact that the inner handwheel can be coupled to the outer handwheel in a detachable fashion by way of an axial detent coupling, the inner handwheel can also be secured in a non-rotational fashion, thus preventing the unintentional release of the axial locking of the blade as well as of the chain tensioning device.
By virtue of the fact that spring means seek to press the inner handwheel and the outer handwheel axially apart from each other, in particular spring means disposed axially between the two handwheels, the blade position is secured when the inner handwheel is loosened without the outer handwheel rotating along with it in an undesired manner and thereby reducing the selected saw chain tension.
By virtue of the fact that the spring means seek to press the cam and the inner handwheel axially apart from each other, in particular spring means disposed axially between the cam and the inner handwheel, the chain tensioning device can form a complete structural unit that can be preassembled in captive fashion, in particular along with the sprocket cover, and can be connected to the housing without errors in assembly.
By virtue of the fact that an inner protective cover closes the chain tensioning device from the inside at the sprocket cover, dust and shavings are prevented from penetrating into the chain tensioning device and thus impairing its function.
By virtue of the fact that the chain tensioning device has an elongated, angular stop plate for gripping the cam, which can be fixed to the blade, the stop plate is particularly resistant to deformation and bending while using a particularly small amount of material, so that the chainsaw is lighter and the chain tensioning device operates in a more directly responsive manner than is the case with known U-shaped stop plates.